ARM: rmobile: beacon: Set CONFIG_RZ_G2 on Beacon boards

The board detection is incorrectly stating it's an rcar3 variant
instead of an RZ/G2 variant on all the r8a774*1_beacon boards.
Set the flag to correctly display as RZ/G2[M/N/H]

Signed-off-by: Adam Ford <aford173@gmail.com>